On the equator, will the temperature change with the terrain?

The surface of the earth is rugged, and its actual shape is very irregular. But compared with the size of the earth, the difference of ground fluctuation can be ignored. Therefore, when discussing the topic of the shape of the earth, in order to make its overall shape characteristics not be covered by the subtle differences of the ground ups and downs, people do not consider the shape of the natural surface of the earth, but study its theoretical surface shape. This is the shape of the global static sea surface.

The so-called global static "sea surface" shape refers to the shape of the sea surface. On the surface, it ignores the difference between land and sea, and the sea surface is obviously much simpler and smoother. The so-called "static" sea surface refers to the average sea surface, assuming that the sea surface is completely calm, without wave fluctuation and tidal fluctuation, and without the influence of ocean currents. The so-called "global" static sea surface includes not only the actual Pacific Ocean, Atlantic Ocean, Indian Ocean and Arctic Ocean, but also the "extension" of the static sea surface below the land in some imaginary way, forming a global closed surface called the geoid. This is the equipotential surface under the action of gravity, and it is the starting surface of altitude on the ground. The shape of the earth refers to the shape of the geoid.

The earth is a sphere.

The earth is a sphere.

Human understanding of the shape of the earth has a very long history. Because the earth itself is huge, but people's vision is very limited, and they can't know the shape of the earth by intuitive feeling. A person standing on the flat ground can only see about 4.6 kilometers away. This small part of the earth looks like an airplane. In ancient China, there was a saying that "the sky is round like a cover and the ground is flat like chess", that is, the sky is round and the ground is flat.

However, many signs show that the ground is not a plane, but a curved surface. For example, you can see far if you climb high. The human eye is about 1.5m from the ground and can only see 4.6km away. If you climb to the height of 1000m, you can see beyond 12 1km. This is a good proof that the ground is curved.

Another example is that when people watch distant ships on the shore, they always see the mast first and then the hull; When the ship leaves port, the opposite is true. First, the hull, then the mast is hidden at sea level. If the earth is flat, the hull and mast should be visible at the same time no matter how far away (Figure 6- 1).

For another example, the height of Polaris changes with latitude, and the farther north it is, the higher its horizon is. In all parts of southern China, people can see this old star in the southern sky, while in the north, this old star is always hidden on the southern horizon. In this way, different places have different horizons, and the ground itself can only be curved. If the ground is flat, then distant stars should form the same altitude angle with all parts of the ground.

All the above phenomena prove that the earth is a curved surface. However, a surface is not necessarily a sphere, and only surfaces with the same curvature can form a sphere. Modern measurement shows that the curvature of all parts of the ground is roughly the same, and each degree is about 1llkm. It can be seen that the conclusion of spherical earth is based on strict inference and accurate measurement. Magellan's voyage around the world only proved that the earth is a closed surface with facts. In today's space exploration, astronauts do see that the earth is a sphere when they take a spaceship or land on the moon.

Figure 6- 1 Intuitive Evidence of Curved Earth

(1) If the earth is flat, the mast and hull of distant ships should be visible at the same time;

(bottom) The earth is curved, and ships coming in the distance see the mast first and then the hull.

60 1-2 Determination of Earth Size

When people realize that the earth under their feet is a sphere, they will naturally ask such a question: How big is the earth?

Measuring the size of a sphere is relatively simple. By measuring the arc length of meridian (geodesy) and its included angle with the center of the earth (astronomical measurement), we can know the total length of meridian circle, and learn radius of the earth and other data. It is easier to determine the opening angle of meridian arc length to the center of the earth. This value can be obtained by comparing the height of the sun at noon in two places on the same meridian on the same day. Is the latitude difference between the two places.

Eratosthenes, an ancient Greek scholar, roughly determined the size of the earth for the first time in history. He knew that at noon on the summer solstice, the sun was at the zenith of Aswan (formerly called Sydney) in southern Egypt, and the sun was directly at the bottom of the deep well. Eratosthenes thinks Aswan is located in the Tropic of Cancer. He also estimated that Alexander and Aswan are located on the same meridian, and the distance between them is about 5000 sight lines (Greece). In this way, he can get the size of the earth by measuring the height of the noon sun during the summer solstice in Alexandria.

Eratosthenes does not directly measure the height of the noon sun, but uses a standard table to measure the length of the noon shadow. This gauge is a semi-hollow ball with a vertical axis in the middle. This axis is the radius of the sphere. When the scale is placed on the ground, this axis is perpendicular to the ground and points to the zenith (Figure 6-3). According to Eratosthenes's measurement, the shadow length of the normal axis projected on the sphere is about the entire circumference of 1/50, that is, about 7.2, during the summer solstice in Alexandria at noon. The ancient Greeks had a fairly comprehensive understanding of geometry. Eratosthenes deduced that the ratio of the shadow length and circumference projected by the gauge axis on the inner surface of the sphere is equal to the ratio of the meridian arc length between Aswan and Alexandria to the circumference of the earth. In other words, the meridian circumference of the earth is equal to 50 times the distance from Aswan to Alexandria, that is, 250 thousand sight distance. L The platform is158m, and the circumference of the earth is 39,500km. This is quite close to the modern measured value of 40025km, which is about 6370km in radius of the earth.

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The shape and size of the earth in the third quarter

Figure 6-2 Measuring the Meridian Length of the Earth

Figure 6-3 Left: The zenith distance of the sun from summer to noon in Alexandria is the meridian between Alexandria and Aswan, that is, the latitude difference between the two places. Right: Eratosthenes's standard for measuring the zenith distance of the sun.

Strictly speaking, Eratosthenes's work of measuring the size of the earth is actually only half finished, that is, measuring the latitude difference between the two places; The distance between the two places is estimated, not measured. The earliest time to measure the meridian length was the astronomer monk and his team (real name Zhang Sui, 673-727) in the Tang Dynasty. In 724 AD, under his auspices, Tai Shijian Nan Gong said that he led a survey team to measure the noon shadow length and the "North Pole Height" (latitude) of Huaxian, Kaifeng, Fugou and Shangcai, which are generally located on the same meridian, and at the same time measured the horizontal distance between the above places, thus obtaining "351 Li and 80 steps, extremely poor degree". There is no concept of a spherical earth on a line. He just used the measured data to deny the saying that "the shadow of the sun is only one inch away", instead of taking "extreme difference of one degree" as the latitude on the ground. Therefore, the line did not understand that it was the determination of the meridian length of the earth, just as Columbus later did not know that the land he discovered was America.

60 1-3 gravity and shape of the earth

The earth is an object with internal gravity balance. According to the symmetry principle, all such objects are spherical.

So many celestial bodies such as the sun, moon and planets are spherical. Copernicus, the founder of modern astronomy, said, "The sphere is the most perfect shape in all things; Because this shape is the largest, everything is suitable. " He also said: "Gravity is just ... a natural tendency to combine objects into spheres". The earth forms a sphere under its own gravity.

The factors that affect the shape of celestial bodies are not only their own gravity, but also the cohesion of solid molecules. The former makes the celestial body tend to be spherical, while the latter keeps the object in its original shape. The magnitude of self-gravity depends on the mass of celestial bodies. All celestial bodies with small mass cannot become spheres, because self-attraction is not as effective as molecular cohesion. For example, meteorites from interstellar space are not spherical. Space exploration shows that the two small satellites of Mars are not spherical. According to the change of brightness, the shape of asteroids is irregular, except for a few asteroids with huge mass. This shows that only a celestial body with huge mass has the shape of a sphere if its own gravity exceeds the molecular cohesion.

However, not all massive celestial bodies are spherical. For example, nebulae are much larger than stars, but they are not spheres. This is because the evolution of celestial bodies, from aspheric surface to spherical surface, takes a time process. Massive celestial bodies are not all spherical at different stages of their development.

The earth is a flat sphere.

The earth is a flat sphere.

A strict sphere is a regular sphere, which has a uniform radius, so it has a uniform curvature and circumference. The earth is not such a sphere, but a flat sphere.

The oblate sphere of the earth was discovered by a pendulum. 1672, a French astronomer, Richard, was sent by the Paris Academy of Sciences to Cayenne, the capital of French Guiana, South America, to observe the parallax of Mars. He brought a high-quality pendulum clock. After arriving in Kayan, Risher found that her clock, which kept good time, suddenly slowed down, which was 2 minutes and 28 seconds every day and night. This is a big mistake. He had to adjust his pendulum clock according to the movement of the stars, shorten the pendulum length by 4 mm, and the pendulum clock resumed normal walking. Two years later, Ricky returned to Paris, only to find that the clock was fast again, and the accelerated value happened to be the value that had slowed down in South America. He restored the pendulum to its original length, so the clock was accurate again. Figure 6-4 The earth is a oblate sphere with an equatorial radius greater than the polar radius and an elliptical meridian. Before that, people thought that the length of the second pendulum should be the same everywhere, and some even advocated using it as the unit of length. In those days, when Luo Li in Cang Li measured the acceleration of gravity, he didn't doubt it. The pendulum slows down near the equator, which can be convincingly explained by the decrease of gravity. However, why does gravity change with latitude? People then associate it with the movement and shape of the earth. This is another leap in understanding.

The characteristics of flat sphere are as follows: the radius of sphere decreases with the increase of latitude: the equatorial radius is the longest and the polar radius is the shortest; In connection with this feature, on the oblate sphere, the equator and latitude are still perfect circles, while the longitude lines are all ellipses, and their curvature decreases from the equator to the north and south poles.

The oblateness of a flat sphere is expressed by oblateness. If the equatorial radius of the earth is a and the polar radius is b, the oblateness (f) of the earth is:

There is a process to improve the accuracy of data on the shape and size of the earth. 1975 In September, the18th plenary meeting convened by the International Federation of Geodesy and Geophysics decided to adopt the following data of 1984:

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Section 13 the shape and size of the earth

Equatorial radius (a) = a)=6 378. 140km km.

Polar radius (b) = 6356.755km.

Flatness (f) = 1/298. 275

602-2 Earth Rotation and Earth Shape

If self-gravity is the only factor that forms a sphere, then the earth must be a regular sphere. However, the earth is a rotating body, and it is also affected by inertial centrifugal force. Every particle on the earth is subjected to the resultant force of gravity and inertial centrifugal force, which is gravity (see 608-L). Risher attributed the change of gravity with latitude to the action of inertial centrifugal force. On the equator, the earth rotates fastest, the inertial centrifugal force is the largest, and the gravity decreases; At the poles, the rotational speed and inertial centrifugal force are equal to zero, where gravity is the largest. The calculation shows that due to the influence of inertial centrifugal force, the gravitational ratio on the equator decreases by l/289 at the poles. However, the actual ground gravity difference is much bigger than this. Gravity difference at equator and poles is1190. Obviously, it is not only inertial centrifugal force that affects the ground gravity.

It was Newton who gave a satisfactory explanation for this. He pointed out that there are two reasons for the decrease of ground gravity from the poles to the equator: one is inertial centrifugal force, and the other is the flattening of the earth. Newton proved irrefutably in theory that the earth itself must be flat under the inertial centrifugal force of rotation.

On the rotating earth, the center of the circular motion of each particle is on the earth axis, and the direction of inertial centrifugal force is perpendicular to the well and deviates from the earth axis. If the inertial centrifugal force in a place is decomposed into vertical and horizontal components, then the latter component points to the equator (Figure 6-5). It is under the action of the horizontal component pointing to the equator that matter tends to gather towards the equator, and the earth becomes a flat sphere.

Figure 6-5 What directly causes the oblateness of the earth's sphere is the inertial centrifugal force of rotation (F). Its horizontal component (f) points to the equator; The vertical component counteracts some gravity to a small extent.

Newton also observed Jupiter and Saturn, and found that they both have the shape of equatorial bulge and polar contraction, thus inferring that the earth must also have this shape. According to the theory of the earth's oblate sphere, Newton successfully explained the causes of the precession of the earth's axis and the precession of the bisector. This is a manifestation of the internal relationship between the movement of the earth and the shape of the earth.

602-3 Geographic Latitude and Geocentric Latitude

When the earth changes from a regular sphere to a flat sphere, there are two different methods to measure the latitude on the earth: one method defines latitude as the angle between the ground normal and the equatorial plane of the earth; Another method defines latitude as the intersection angle between the radius of the earth and the equatorial plane. The former emphasizes the degree of an arc from the equator along the local meridian to the location, which is called geographical latitude; The latter emphasizes that the central angle from the arc to the center of the earth is called geocentric latitude.

Figure 6-6 On a flat sphere, the radius of the sphere only passes through the center of the sphere and is not perpendicular to the sphere; The normal is only perpendicular to the sphere and does not pass through the center of the sphere. So latitude is divided into geographical latitude (J) and geocentric latitude (J'), where J > J'.

Figure 6-7 The difference between geographical latitude and geocentric latitude is the largest at 45 latitude.

Speaking of geographical coordinates, we regard the earth as a regular sphere. On the right sphere, the ground normal coincides with radius of the earth, so there is no difference between the two latitudes. But in fact, the earth is a flat sphere. On a plane sphere, except the equator and poles, the straight line perpendicular to the ground does not pass through the center of the earth; On the contrary, the straight line through the center of the earth is not perpendicular to the ground. So there are two latitudes; Because the meridian curvature of a flat sphere decreases from the equator to the poles, the geographical latitude of a place is always greater than its geocentric latitude (Figure 6-6).

The difference between geographical latitude and geocentric latitude itself changes with latitude. At latitude 45 degrees north and south, the difference between the two latitudes is the largest (11'32 "), and decreases to zero towards the equator and the poles (Figure 6-7). We know that the curvature of the meridian decreases from the equator to the poles, and the meridian curvature at 45 north and south latitude can be regarded as the average curvature of the meridian. Compared with it, the curvature of this meridian is greater than the average curvature of 45 from the equator to the north and south latitude, so its geographical latitude is greater than the geocentric latitude, and the difference between them continues to increase with the increase of latitude. On the contrary, the curvature of this meridian from 45 to the north and south poles is less than the average curvature, and the difference between the two latitudes begins to decrease from 45 to the north and south poles, and the cumulative difference decreases to zero. In other words, 45 degrees north and south latitude is the end point of the continuous increase of the difference between the two latitudes, and it is also the starting point of the continuous decrease. Therefore, there is a maximum and a minimum at the equator and poles.

Geographical considerations are mainly about how the horizon of each place is different from that of the equator, not the direction of the center of the earth.

Therefore, it applies geographical latitude in principle. In general, this slight difference can be ignored.

The earth is an irregular oblate sphere.

The earth is an irregular oblate sphere.

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Section 13 the shape and size of the earth

A flat sphere is not as simple as a regular sphere, but it has strict rules in geometry. Its latitudes are all perfect circles, and its meridians are all ellipses. Such a sphere can be regarded as an ellipse rotating around its short axis, so it is also called a rotating ellipsoid.

Strictly speaking, the true shape of the earth (geoid) is not a geometrically rotating ellipsoid. Its shape is irregular: latitude is not a strict circle, and longitude is not a real ellipse; The northern and southern hemispheres of the earth are asymmetrical, and its geometric center is not on the equatorial plane. The earth is an irregular oblate sphere. Such an irregular sphere cannot be expressed by simple geometric or mathematical methods, so people use it to compare with an ideal "model" to illustrate it.

In order to show the irregularity of the earth's shape, we can imagine a reference oblate sphere. It has the strict regularity of a flat sphere and its shape and size are very close to the geoid. Say it is a reference "flat sphere" because it is a strict flat sphere; It is called the "reference" hemisphere because it represents the basic aspects of the shape of the earth. In fact, all the data of the earth's oblate sphere mentioned above refer to the data of oblate sphere. Figure 6-8 Deviation between the geoid (solid line) and the reference oblate sphere (dotted line) (no conclusion can be drawn from this:

The earth is shaped like a pear. With the reference oblate sphere, the true shape of the earth can be expressed by the deviation of each part of the geoid from the reference oblate sphere in meters. Figure 6-8 shows the longitudinal section of the geoid. As can be seen from the figure, the maximum deviation of geoid is only tens of meters compared with its nearest oblate sphere. Generally speaking, the geoid in high latitudes in the northern hemisphere and low latitudes in the southern hemisphere is higher than the reference oblate sphere; However, in the low latitudes of the northern hemisphere and the high latitudes of the southern hemisphere, the geoid is slightly lower than the reference oblate sphere. The obvious contrast is the difference of polar radius between the northern hemisphere and the southern hemisphere: the geoid of the Arctic is about10m higher than the reference oblate, while the geoid of the Antarctic is about 30m lower than the reference oblate. The difference between the two is 40 meters. Comparatively speaking, the northern hemisphere is slightly convex and the southern hemisphere is relatively flat.

Figure 6-8 Deviation between the geoid (solid line) and the reference oblate sphere (dotted line) (it cannot be concluded that the earth is shaped like a pear)

The shape of the geoid shown in Figure 6-8 once gave people a pear-shaped impression. As soon as this picture came out, people rumored that the saying of "pear-shaped soil" spread like wildfire. In fact, the figure is only used to show that the geoid in the northern hemisphere and the southern hemisphere have different deviations from the reference oblate sphere, and it cannot represent the real shape of the earth. This is because: firstly, in order to highlight this deviation, it has to ignore the "oblateness" of the reference oblate sphere, the huge difference between the equatorial radius and the polar radius of the earth at nearly 2 1km, and replace the ellipse with a perfect circle; Secondly, the deviation between the geoid and the reference oblate sphere is exaggerated. The scale used to represent the 40m difference between the north and south poles is 57,000 times larger than that used to represent radius of the earth. Because of this exaggeration, the geoid is shaped like a pear. In fact, for the earth with a long radius of 6378. 140 km and a flat chord of 1/298.275, the difference of 40 meters between the north and south poles is very small. It only changes the curvature of various parts of the ground slightly. Any part of the geoid is convex, without depression and without edges and corners.

603-2 Distribution of Matter in the Earth and the Shape of the Earth

The gravity of the earth and the inertial centrifugal force of rotation are both systematic factors. Under their influence, the shape of the earth must be regular. Since the shape of the earth is irregular, there must be non-systematic factors at work. This factor is the uneven distribution of matter in the earth.

The composition and density of matter in the earth vary with depth. Usually, we always think that the earth is composed of uniform concentric spheres. But strictly speaking, because the differentiation of matter in the earth has not been finally completed, the spheres in the earth are neither truly homogeneous nor truly concentric. This situation will inevitably affect the gravity distribution on the ground and the shape of the geoid. For this reason, the center of mass of the earth is not located in its geometric center.

The uneven distribution of matter is particularly obvious in the crust. There are differences and ups and downs on the surface of the earth. As mentioned above, the shape of the earth refers to the shape of the sea surface, ignoring the difference between the land and sea distribution of the surface and the topographic relief. This is to simplify the shape of the earth and exclude these factors from geometry. However, the shape of the sea itself has to be physically influenced by the difference between land and sea and the height of the terrain. The fluctuation of the ground is not only a problem on the ground, but also reflects the distribution of underground substances to some extent. And where the mountain is located, there is not much underground material; Where the deep sea is located, there is not necessarily a particularly small amount of material underground. All these complicated conditions will affect the geoid and make the oblate sphere of the earth into an irregular shape.

Geometrically speaking, the shape of the earth is irregular. However, in the physical sense, the shape of the earth is regular. This is because the global static sea surface, no matter how complicated in geometry, is always an equipotential surface. On this equipotential surface, objects have the same gravitational potential energy.

The shape of the earth's sea surface is related to the gravity distribution on the ground. The inertial centrifugal force of rotation affects the ground gravity, making the earth change from a regular sphere to a flat sphere. Similarly, the uneven distribution of materials in the earth causes local differences in surface gravity, thus forming local fluctuations in the sea surface. On the sea surface with large gravity acceleration, the sea surface itself must be geometrically low; On the contrary, on the sea surface with small acceleration of gravity, the sea surface itself must be geometrically higher. Only in this way can the sea surface be an equipotential surface. Otherwise, the seawater will flow, and the result of the flow will inevitably be the formation of equipotential surface, that is, the geometric regularity of the earth's shape will be destroyed. Therefore, the geometric irregularity and physical regularity of the earth's shape are mutually conditional.

Review and thinking

● What is the geoid? How do people know that the ground is not a plane but a curved surface? How do you know that the earth is a sphere?

Why did the earth become a flat sphere? Comparing geographical latitude with geocentric latitude, why is geographical latitude greater than geocentric latitude? Why is the difference between latitude 45 north and south the biggest?

● What is a reference oblate sphere? How do the parts of the geoid deviate from the reference oblate sphere? Why is it inaccurate to say in general that "the earth looks like a pear"?

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There are two photos of the earth on the last page. One is the earth seen on the moon (the earth also has a phase change); The other is a photo of the earth taken by a synchronous satellite at a distance of 36,000 kilometers. This photo shows less than half the world, so Africa occupies a larger area than it really is.

(1) see "New Tang Book Tian". According to the length unit of the Tang Dynasty, 1 is 300 steps, the L step is 5 feet, and the circumference is 365.25, which is converted into modern units, about 12922km, which is quite different from the modern measured value.

(1) Copernicus, On the Operation of Celestial Bodies, Beijing: Science Press, pp. 1973.8 and 26.

① Inertial centrifugal force is different from centrifugal force. It is just a kind of vision, a manifestation of the inertia of circular motion, and a tendency to leave the center of the circle. Centrifugal force and inertial centrifugal force act on different objects. For example, if a stone is tied with a rope and one end of the rope is held by hand to make it rotate, then the centrifugal force is the force of the stone opponent (the force of the hand on the stone is centripetal force), and the inertial centrifugal force is the force acting on the stone.

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6 Section 14 Structure of the Earth

Section 14 Structure of the Earth

The spherical structure of the earth

The earth is a heterogeneous sphere. In the process of long-term movement and material differentiation, according to different densities, it is separated into several concentric (geocentric) spheres composed of different States and different substances.

604- 1 the external structure of the earth

The surface of the earth is made of rocks. Mainly magmatic rocks, that is, rocks condensed from hot magma rising from the ground. At the top of magmatic rocks, there is usually a thin layer of sedimentary rocks (generally less than 4-5 kilometers) and a thinner soil cover. This surface layer of the earth, including magmatic rocks, sedimentary rocks and soil cover, is called lithosphere.

Most of the surface of the lithosphere is covered by the ocean. In low-lying places, liquid water will also stop flowing and become lakes and rivers. In the mountainous areas of land (high latitudes and mountains), solid water accumulates into glaciers. In addition, there is groundwater at a certain depth below the surface. All these different forms of water form a hydrosphere.

Above the lithosphere and hydrosphere, the whole earth is surrounded by an atmosphere with nitrogen and oxygen as the main components, which is called the atmosphere. The atmosphere is the outermost layer of the earth and the transition layer from the ground to interplanetary space. There is no clear upper limit.

70%-75% of the atmosphere is concentrated in the bottom layer 9 km (poles) to 17 km (equator) thick. The main feature of this atmosphere is to get heat and moisture from the ground, so there are convection and weather phenomena, which are called troposphere.

Lithosphere, hydrosphere and atmosphere are not only separated and independent from each other, but also penetrate and interact with each other. In this way, there is a zone with both minerals, air and moisture on the earth, which becomes a biologically derived zone called the biosphere when the temperature conditions are suitable. It includes the upper lithosphere, the bottom of the atmosphere and the whole hydrosphere, and is a unique lithosphere on the earth.

The exterior of the earth consists of lithosphere, hydrosphere, atmosphere and biosphere.

604-2 Seismic Wave and the Internal Structure of the Earth

People can directly observe the external structure of the earth; However, it is much more difficult to study the situation inside the earth, because what people can directly observe is only the rock and mineral samples exposed on the surface and in drilling. But at present, the deepest borehole in the world is only about 10km, which is extremely limited for the average radius of the earth of 637 1km. In this way, the internal structure of the earth can only be studied by various indirect means, such as the propagation of seismic waves and the conduction of heat, magnetism and gravity. Among them, the propagation of seismic waves is the most important for studying the internal structure of the earth. Just as doctors use ultrasound to check the lesions of human internal organs, geophysicists use seismic waves to detect the internal structure of the earth. There are two sources of seismic waves, artificial explosion and natural earthquake. Artificial seismic waves have low energy and are widely used to detect stratigraphic structures and underground ore bodies. Strong natural earthquakes, seismic waves spread from the source through the earth's medium in all directions to the whole earth.

Seismic waves are elastic waves, which are divided into body waves and surface waves. Body waves spread from the earthquake source in the earth to the whole world, just like lights illuminating every corner of the room. Surface waves spread in all directions from the epicenter along the surface, just like water waves aroused by riprap water. Body waves are directly related to the internal structure of the earth.

Seismic body waves are divided into longitudinal waves (P waves) and shear waves (S waves). P wave is a kind of compression wave, which is the reciprocating motion of material particles along the wave propagation direction, which makes the medium compress and expand periodically (Figure 6-9). For example, the relaxation of the media can be imagined as the opening and closing movement of the accordion. This kind of seismic wave can propagate in any medium. Shear wave is a kind of shear wave, which is the vibration of material particles perpendicular to the propagation direction of Apollo, just like the familiar snake. The wave produced by shaking the rope is similar to S wave, which makes the medium deform periodically. This shock wave cannot pass through liquid and gaseous media. Longitudinal wave travels faster than shear wave, and it always reaches the station before shear wave. According to the lag time of shear wave, the location and distance of the source can be inferred.

The reason why seismic waves can reflect the physical properties of the earth's interior is that its propagation speed changes with the elasticity and density of the materials in the earth. Due to the change of seismic wave velocity, seismic rays are refracted; Where the velocity of seismic waves suddenly changes, seismic rays will still be reflected. Refraction and reflection make seismic rays change from straight lines to curves and broken lines. Therefore, the propagation of seismic waves is very complicated; The complex propagation of seismic waves is caused by the internal structure of the earth.

Figure 6-9 P-wave (top) and S-wave (bottom)

Inside the earth, the velocity of seismic waves varies with depth. This is because the density and elasticity of underground materials vary with depth. In different places at the same depth, the wave velocity is the same. In this way, underground matter can be divided into different circles according to depth, that is, density and elasticity. This is the inner ring structure of the earth.

Figure 6- 10 Sphere Structure in the Earth

Figure 6- 1 1 Wave speed of seismic wave

604-3 Internal Structure of the Earth

According to the study of seismic wave propagation, the earth's interior is divided into four main circles. They are the crust, mantle and core; Core is divided into outer core and inner core (Figure 6- 10). There is a physical interface between layers, that is, a discontinuous surface. The interface between the crust and mantle is 20-30 kilometers underground, which is called Moho surface. There, the wave velocities of P wave and S wave increase sharply. The depth of the interface between mantle and outer core is about 2 900km, which is called Gutenberg discontinuity. There, the speed of P wave drops sharply, and S wave stops and disappears suddenly. The interface between the outer core and the inner core appears at the depth of 5 100km, which is called Rehmann discontinuity. On this interface, P wave accelerates sharply again, and S wave appears again (converted from P wave).

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7 Reply 6: Section 14 Structure of the Earth

According to the model proposed by seismologist K E Bullen in 1970, the depth and thickness of the crust, mantle and core are as follows:

Post related pictures:

Author: Yun Mengpu 2006-3-26 18:0 1 Reply to this statement

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8 Reply 7: Section 14 Structure of the Earth

The thickness of the crust is very uneven. Part of the continental crust is thick, with an average of about 30km；; Some oceanic crust is thin, with an average of 1 1Km, and the thinnest part of the Pacific Ocean floor is only 8km. There is another time in the earth's crust.